The Green Party has thrown down the gauntlet to the other party leaders over their seeming exclusion from the televised election debates asking them: "What are you afraid of, boys?"

The new poster campaign comes as party leader Natalie Bennett is pushing to be included in the leaders debates, after media regulator Ofcom ruling in an initial judgement that the Greens were not a "major party" in the same way Labour, the Tories, the Lib Dems and Ukip are.

The Greens' new election poster

Bennett confronted Nick Clegg over the weekend, urging him to get his "other two amigos" Nigel Farage and Ed Miliband to also write to the broadcasters to ask for the Greens to be included.

But the Liberal Democrat leader insisted it is for the broadcasters to come forward with new proposals due to concerns over the existing plans.

Nick Clegg clashed with Green leader Natalie Bennett

Speaking on BBC One's the Andrew Marr Show, Clegg reiterated he believes the debates will take place.

He claimed the Tories are "swithering around a lot", adding: "I think it's inevitable in a hugely fragmented political environment - fragmented in a way I can't remember in my political lifetime - that two big vested interests, the red team and the blue team, in this case using the Greens as an alibi on the debates, will try and squeeze other voices out.

"They want to put the genie back in the bottle. They want to run things as they've done before. I think it's too late."

Bennett then told Clegg: "I think we've spent most of the last week debating about the debates instead of talking about issues and I don't think the public is enjoying that or wants that to be happening.

"So Clegg, will you perhaps get together with your other two amigos, Farage and Miliband, and write to the broadcasters and say include the Greens as the public wants and then we can get on to debating the issues instead of debating debates?"

Clegg replied: "The broadcasters need to come forward with other proposals because clearly the current one, which I am not wildly happy about because it excludes me as a leader of a governing party, so they need to come forward with proposals. I'll get my soapbox out any day of the week."

On Thursday morning, Green Party sources said it had 43,829 members as of midday. Ukip's reported membership is 41,966. Green Party sources also claimed to have put on 2,000 members the day before, in what potentially is its biggest one-day surge in numbers ever.

The Green Party currently has one MP, Brighton Pavillion's Caroline Lucas. It will hope to cling on to that seat in 2015 and possibly add another in Bristol.

Ukip currently has two MPs, Tory defectors Douglas Carswell and Mark Reckless. Nigel Farage hopes to get at least a handful more in May.

A Green spokesman for the party told HuffPost its membership figures and latest polling "further calls into Ofcom's initial view that the green party is not a 'major party'."

David Cameron has said he will refuse to take part in the debates if the Greens are not involved. However the other party leaders have accused him of using that as an excuse in an attempt to run away from the debates.

Nick Clegg said last week that the idea the prime minister really cared about Bennett being allowed onto the stage was "laughable".

However, George Osborne insisted Cameron was "up for" televised general election debates - but only if the format is right.